A hollow fiber membrane module is a filtration device using, as separation membranes, hollow fibers which are thin fibers, the surface of which has a myriad of small pores, and the central space of which is empty. This module is configured such that, among a multi-component liquid mixture, a material larger than the pores is excluded, and only a specific material smaller than the pores is selectively passed therethrough and thus effectively recovered. Such a hollow fiber membrane module has been widely utilized in ultrafiltration or microfiltration for preparing sterile water, drinking water, etc., and is recently applied to secondary or tertiary treatment in sewage treatment plants, or to solid-liquid separation of purification tanks. The hollow fiber membrane module used for sewage treatment is classified into an external circulation mode and a module immersion mode, depending on the type in which hollow fiber membranes are applied to aeration tanks.
When a partition membrane is provided at the lower portion of a submerged membrane module, an upward water flow is formed, resulting in cross-flow. As such, when the distance between the lower header and the partition membrane of the module is increased, the upward water flow does not circulate in bundles of hollow fibers which are densely packed. Also, in the case where there is no space between the lower header and the partition membrane, the size of a raw water supply hole of the lower header is limited by the space which is occupied with the bundles of hollow fibers, and thus a sufficient upward water flow is not formed, and operating efficiency may decrease in a reverse washing process.